[Characteristics of chord time constant and F-V curve configuration in testing small airway functions for smokers]

To study the significance of chord time constant (tau ch) and F-V curve configuration in testing small airway function for smokers, the routine ventilatory functions, chord time constant (tau ch) and F-V curve configurations were studied in 116 passive and active (light, heavy) smokers. The tau ch was calculated with Knudson's method, tau ch = V/V.

The distribution of convex type rose with the degree of smoking. The tau ch50 in heavy smokers and convex F-V curve prolongated to 130% of predicted value; tau ch50 in light smokers prolongated to 120%. These reflected that there were different degrees of disturbance in small airway functions of smokers and suggested that tau ch50 should be a sensitive and specific index for testing small airway functions. On the other hand, the tau ch75, tau ch50 and tau ch25 by the present authors previously measured in patients with chronic obstructive pulmonary diseases (COPD) all prolongated with the severity of the disease, which reflected that both large and small airways were affected. Therefore it is concluded that in terms of clinical implications the specificity of tau ch50 for small airway functions and the prolongation of tau ch with the severity of COPD are useful indexes.